A known type of a fuel cell for generating electricity by utilizing electrochemical reaction between hydrogen and oxygen is a solid oxide fuel cell (hereinafter, also called “SOFC”) which has electrolyte layers containing solid oxide. A fuel cell electricity generation unit (hereinafter, may be called merely “electricity generation unit”), which is the smallest unit of electricity generation of SOFC, has a single cell which includes an electrolyte layer and mutually facing cathode and anode with the electrolyte layer intervening therebetween, and electrically conductive current collectors disposed respectively on a cathode side and on an anode side of the single cell and adapted to collect electricity generated in the single cell. Generally, the current collector disposed on the cathode side of the single cell has protrusions protruding toward the cathode. The cathode and the protrusions of the current collector are bonded together by an electrically conductive bonding layer, whereby the cathode and the current collector are electrically connected.
The current collector disposed on the cathode side of the single cell is formed of, for example, a metal which contains Cr (chromium), such as ferritic stainless steel. When such a current collector is exposed to an atmosphere of a high temperature of, for example, 700° C. to 1,000° C. in the course of operation of SOFC, there may arise a phenomenon called “Cr diffusion” in which Cr is emitted from the surface of the current collector and diffused. When Cr emitted from the current collector enters the bonding layer, unfavorably, the emitted Cr reacts with a component of the bonding layer to thereby cause deterioration in electric conductivity of the bonding layer, or passes through the bonding layer to reach the interface with the cathode, causing a phenomenon called “Cr poisoning of cathode” in which the electrode reaction rate in the cathode deteriorates. In order to restrain Cr diffusion from the current collector, there is known a technique in which the surface of the current collector is covered with an electrically conductive coat (see, for example, Patent Document 1).